Image screen for braun tubes



June 27, 1939. SCHWARTZ 2,1fi3,918

IMAGE SCREEN FOR BRAUN TUBES Filed June 6, 19:56 2 Shee ts-Sheet 1 g My4/ Elm/K WMZY v June 2'7, 1939. E. SCHWARTZ IMAGE SCREEN FOR BRAUN TUBESFiled June e, 1936 2 Sheets-Sheet 2 Patented June 27, 1939 UNITE STATSIMAGE SCREEN FOR BRAUN TUBES Application June 6, 1936, Serial No. 83,977

. In Germany June 14, 1935 2 Claims.

The present invention relates to cathode ray tubes used for delineatingtelevision images, oscillograms and the like, and deals moreparticularly with an image screen adapted for rendering the delineatedimage visible in incident light or by projection.

Sell-luminous image screens are normally used in Braun tubes, especiallythose embodying predou'iinantly fluorescent materials. It is also knownto make the screens of very finely distributed metal, for example verythin foils or metal gauze. Th se metal screens when bombarded by thecathode ray become very strongly locally heated, the heating dependingon the intensity of the ray, so that they are suited for delineatingtelevision images. Both types of screens, including the fluorescentscreen as well as the metal screen acting as a temperature responsiveradiant body however, have the drawback that they possess a luminouspower which is limited by the capacity of the cathode ray and theelectro-optical network efiect of the screen.

In accordance with the present invention a self-luminous screen isentirely avoided and an image screen is used which alters its opticalproperties (such as color, reflecting power or light transmissibility)under the influence of cathode ray bombardment, so that the televisionimage will not appear as a luminously contrasty image but rather mustwhen viewed, be lighted by an outside light source or by daylight. Thisuse of an outside light source completely eliminates the lightingproblem so that it is possible to do the viewing in undarkened rooms orto project (the images) in desired size either episcopically ordiascopically.

The subject matter of the invention is described below and illustratedin the drawings, wherein:

Figs. l-3 show a graphical representation of the behavior of such ascreen.

Fig. 4 is a cross section through the screen.

Fig. 5 is a graphical representation of the relations betweentemperature and color of such a screen.

Figs. 6 and 7 show two embodiments for episcopic and diascopicprojection.

In order to produce a variable color image screen, a coloring agent isused the color of which depends on the temperature and in respect ofwhich the change of color occurs either suddenly or within a definitetemperature range. Substances which are suitable for this purpose are:silver-mercury-iodide, copper-mercury-iodide, or

a mixture of both. The transition temperatures of these coloring agentslie between 40420" and vary according to the mixture proportions used.Even the temperature range in which the change occurs may be more orless sharply defined by suitable mixing.

If the individual image point were heated only at the instant when thecathode ray bombards it, that is, if it shows a color change only duringthis short period, a very dull image would result; for, if the imagepoint has the color corresponding to a given high temperature onlyduring the image point interval and if during the entire remainingperiod it has the color corresponding to a lower temperature, then theeye would see a mixed color which would possess only a very smallfragment of the heat color desired and which would be appreciablyoverbalanced by the cold color. The desideratum to be sought is thateach image point should retain the color imparted to it by the cathoderay until shortly prior'to the period in which the cathode ray strikesit anew, but nevertheless, prior to the instant when the freshbombardment of the ray occurs, it should suddenly return to its startingvalue. This condition, in accordance with the invention may be attainedby the. use of materials which change their color suddenly in accordancewith temperature and which as a result thereof are preeminently suitedfor the reproduction of black-white images.

Fig. 1 shows the temperature curve of a particle of the screen inrelation to time. Such a screen is, during its operation, preheated tooperating temperature a, e. g. by means of a separate heat source, sothat a small temperature rise. produced by the cathode ray will causethe color change to occur at temperature b. By

. suitably selecting the heat inertia of each screen particle theexponential temperature drop of the screen portions may be so adjustedthat immediately after an image change if the screen portion will passthrough the temperature at which the color change occurs, so that inthis way a sudden and optimum color change will occur.

Such a screen may also be utilized for reproducing images havinghalftones if the relations of the time ranges during which the screenparticles show one or the other color are altered. If for example,according to Fig. 2, during the first image change. interval it, thetemperature is raised to point 0, the image point will appear in thehigh temperature color. If during the next bombardment the temperatureis raised only to d, then the image point will appear as a halftone tothe eye it the time ts be sufficiently small and finally, if the heatingis very weak, say at point e, the image will have the basic color of thematerial.

A further method of producing halftone images consists in usingmaterials the color change of which occurs in a greater temperaturerange (Fig. 3) so that all tonal values between the color values a-g maybe produced by the various cathode ray intensities and the therebyresulting varying degree of heating. In this case the image will alwayssuffer a certain loss of contrast value because the temperature of thescreen particles will drop exponentially during an image alteration.Nevertheless even here an optimum contrast value will be achieved if thedrop is so proportioned that the light image screen portions again reachtheir normal temperature after an image alternation (Fig. 3).

In order to regulate the heat inertia of the screen portions to theabove described extent, the heat insulation of the individual parts issuitably influenced. For example, as shown in Fig. 4, a looseintermediate layer 2 of quartz powder is disposed between glass supportI and the image screen 3 thus improving the insulation of the imagescreen portions as between each other and their glass support. In orderto decrease the quantity of heat insulation used the material may forexample be embedded in a binder such as waterglass.

As discussed in the case of materials which change color suddenly it isalso preferable in the case of materials which change color gradually topreheat the screen by separate heat sources. The screen is then broughtto the temperature T1 (Fig. 5) at which the color change starts so thatthe additional heating caused by the cathode ray governs the color ofthe image screen,

.this procedure starting at the lower kink in the color curve.

The maximum ray output is thereupon so adjusted that it effects anincrease in temperature which extends to the upper kink of the colorcurve T2 that is, the maximum excess temperature of the bombarded imagepoint at a given time must be equal to the color change interval. Thiscondition can be particularly easily attained by using Braun tubes ofthe supplementarily accelerated type and by regulating the supplementaryaccelerating voltages to bring the maximum prevailing ray output up tothe desired value. The necessary pheheating of the image screen may beeffected by heated fluid baths as shown in Fig. 6. Here 4 represents theBraun tube,- 5 the image screen and 6 a water bath heated by immersionheater 1. When dealing with an arrangement intended for projecting theimages it is preferable to use the light source 8, employed forprojection, for heating the screen surface also.

In the case of a coloring agent in which a dark coloration represents ahigh temperature and a light coloration a low temperature, it isnecessary to make the image impulses negative at the Braun tube since inthe case of such colorations the image occurs in reversed relation onthe image screen.

Another embodiment of the invention, shown in Fig. 7, involves anarrangement for diascopic projection and uses for that purpose an imagescreen of which the luminous transparency varies with the energy of theincident ray. For this purpose the glass bottom 9 of a Braun tube I iscoated with a transparent and conducting layer H e. g. of cadmium oxide.Upon this layer is disposed a mosaic of double retracting crystals H.For this purpose small crystals of Rochelle salt (sodium potassiumtartrate), quartz, or similar substances are suitable. It is preferableto allow these crystals to become deposited directly on the bottom ofthe tube from out of a mother liquor poured into the Braun tube. Thesetiny crystals thereby orient themselves in astonishing agreement withrespect to their electrical axes and at the same time make good contactwith the metallic support. If such a mosaic screen is bombarded by acathode ray the crystal surfaces directed away from the metallizedcoating become negatively charged with respect to the metal coating andproduce an electrical field in the crystals the strength of said fielddepending upon the cathode ray intnsity. In this manner the crystalstructure becomes double refracting to an extent corresponding to theincoming image signal. By suitable crystal-optical arrangement such animage screen may be used for transparency projection. Fig. '7 shows aprojection wall l2 which is illuminated by light source l3. The lightpasses through Nicol prism l4 acting as polarizer, then the image screenI1, and is reflected at the black glass plate l so as to illuminate theprojection wall I2. The lens l6 throws the crystal mosaic on the imagescreen.

I claim:

1. A cathode ray device having a light transmitting image screencomprising a support, a layer of screen material including amercuryiodide and an intermediate layer of poor heat conducting materialbetween said support and said layer or screen material.

2. A cathode ray device having a non-luminous light transmitting imagescreen comprising a support, a layer of screen material includingcopper-mercury-iodide and an intermediate layer of poor heat conductingmaterial between said support and said layer of screen material.

ERICH SCHWARTZ.

